1. Field of the Invention
The invention relates to aqueous, solvent-free, polyester- and polyacrylate-based coating compositions and the use thereof.
2. Statement of Related Art
A reduction of the contents of organic solvents in coating compositions is generally desired. There is great interest in aqueous surface coating systems in the field of coating and painting, especially so in the area of automotive painting, but also in other areas.
In the production-line painting of automotive bodies, multi-layer coatings according to the so-called "base coating/top coating" process are preferred. Therein, first a pigmented base coating is applied which, after some short drying time without a baking step, is covered with a clear-coat composition. Then the base coating and clear-coat composition in combination are subjected to baking. This procedure results in coatings having an optically and technologically very high quality.
As far as primers and base coatings are concerned, aqueous systems for a long time have belonged to the state of the art, and in part the use thereof has become a standard operation in the automobile industry. Thus, it is generally desirable to use an aqueous system also for the topmost layer, i.e., the clear-coat.
To serve this purpose, such an aqueous clear-coat composition with respect to its profile of properties must at least be a match for the conventional clear-coat compositions dissolved in organic solvents. The solid contents at the spray viscosity must be high such as to provide sufficient layer thicknesses by one or two spraying steps. Furthermore, the clear-coat films when baked must be well leveled and exhibit high gloss. In addition, the film should be clear and be "full" and have a good "standing".
Also important is reparability. Repair painting to form a well-adhering gravel-resistant coating must be possible, most desirably without grinding by using any conventional base coating, on a first paint already formed by final baking. Furthermore, there must be constituted a good resistance against chemical attacks such as, e.g., against fuels, acids, in particular sulfuric acid (battery acid) or lyes. Very important is resistance to condensed water.
From an ecological perspective, it is desirable to provide surface coating systems which do not contain any organic solvent in order to achieve that during processing and baking environmentally polluting emissions occur practically no longer.
DE-OS 31 28 062 describes emulsion polymers which, according to the application Examples 3 and 4 provided therein were said to be suitable in combination with conventional metallic base coatings for producing weather-resistant two-layer coatings. The requirmenets for reparability and resistance to condensed water during 240 h at 40.degree. C. constant climate (DIN 50017), in combination with aqueous base coatings are not attained.
In EP-A-0 391 271 there have been described aqueous coating compositions which contain, as the main component, a polyester oligomer polyacrylate produced by a free-radical polymerization of ethylenically unsaturated monomers in a hydroxy-functional polyester oligomer which is not polymerizable via a free-radical polymerization.
DE-OS 40 27 594 describes aqueous clear-coat compositions which contain, as binders, emulsion polymers which have been produced with the concomitant use of specific fluorinated ethylenically unsaturated monomers.
All these applications disclose coating compositions which contain relatively large amounts of organic solvents.
Accordingly, it was one object of the present invention to provide aqueous, solvent-free coating compositions which satisfy the requirements of a top coat for automotive bodies and which are, in additional, almost emission-free.